1. Field of the Invention
The present invention relates to a method and to a device for evaluating flow parameters and electric parameters of a porous medium, with the same equipment, the same experiment and from the same rock sample taken from this medium. It can be, for example, rocks from an underground fluid reservoir zone (hydrocarbon reservoir, aquifer, etc.). More particularly, the invention relates to the measurement of the relative permeability and of electrical parameters (resistivity) of a porous medium by subjecting a sample of this medium successively to one or more drainage or imbibition stages.
2. Description of the Prior Art
Starting production of an oil-bearing field therefore requires precise study of the hydrocarbon recovery conditions. It determines, on the one hand, the amount of hydrocarbon in place and, on the other hand, the multiphase flow parameters in the rock that makes up the reservoir, notably during water or oil injection stages.
The rocks that make up a reservoir can be of different natures in terms of petrophysical properties and it is commonplace to divide them into groups referred to as “rock types” so as to work on samples belonging to the same family or rock type. For each family, it is thus necessary to carry out all of the petrophysical measurements to parametrize the simulator, calculate the amounts in place, etc., because these various parameters vary from family to family. For a given rock type, it is thus commonplace to carefully select several samples, one being used for determination of the capillary pressure curves (Pc), the other for the relative permeability curves (krs), another for the resistivity curves, etc.
Determination of multiphase flow parameters such as the relative permeability and the capillary pressure also is a major stage within the context of oil production, whether in the reservoir evaluation stage or when starting production. The relative permeabilities (krs) are used in numerical simulators to describe multiphase flows in the rock, in particular when water or gas is injected to maintain the pressure and recover more oil. These curves typically allow controlling the production profile in reservoirs. Laboratory experiments carried out under reservoir pressure and temperature conditions are commonly conducted to determine these parameters in a representative manner. In particular, the relative permeabilities are conventionally obtained by means of displacement experiments such as those described in French Patent 2,708,742 and corresponding U.S. Pat. No. 5,679,885. These multi-flow displacement experiments inject a fluid (referred to as injected fluid) into a rock sample initially saturated with predominantly another fluid (referred to as displaced fluid). The injected fluid then drains part of the saturating fluid out of the sample. The amount of displaced fluid thus produced by the sample is measured in a volume. This volume is referred to as displaced fluid production volume. As injection is continued, the amount of displaced fluid produced increases, that is the production volume of displaced fluid increases. Stabilization of this volume is reached thereafter, that is the volume no longer increases. This corresponds to a stop in the production of the fluid displaced out of the sample. The injection rate is then increased to drain part of the saturating fluid still in place in the sample, until a new stabilization step is reached. The injection rate is thus increased several times (after each stabilization step) and, for each one of these steps, the evolution of the following parameters, which make up the experimental data, is measured: differential pressure, for one of the fluids, on either side of the sample, denoted by ΔPi(t), displaced fluid production volume, denoted by V(ti), local saturation for one of the two fluids (injected or displaced fluid) and for each injection stage, denoted by S(t1),S(t2), . . . . The injected fluid can be, for example, water, and the displaced fluid can be oil. The relative permeabilities are then obtained after a stage of analytical interpretation of the experimental data, as well as a numerical stage using a flow simulator, so as to take into account all the physical phenomena (capillary pressure, gravity) that influence the experimental data obtained. Such an interpretation is for example described in the above-referenced French patent 2,708,742 and corresponding U.S. Pat. No. 5,679,885.
These patents also describe a device allowing carrying out this type of multi-flow displacement experiments.
As regards determination of the electric parameters, the most common approach also carries out displacement experiments at a fixed flow rate or pressure. The resistivity of a solid sample can be measured by means of electrodes that are in contact with the surface thereof, at selected points, and between which an electric current is passed. The measurement of the potential difference appearing between the locations of the electrodes directly gives the resistivity measurement. Contact of the electrodes with the surface has to be the best possible for the measurements to be representative. A known method places the sample to be tested in an elastic flexible sheath. The electrodes are arranged between the sample and the sheath, and connected therethrough, by electric conductors, to an electrical conductivity measuring system. The sheath is placed in a containment vessel. A fluid under pressure is allowed into the vessel, which has the effect of pressing the sheath and consequently the electrodes against the sample. Such a method using a fluid under pressure for pressing a sheath against a sample is used for example in the petrophysics tools described in French Patent 2,708,742 and corresponding U.S. Pat. No. 5,679,885 and French Patent 2,724,460 and corresponding U.S. Pat. No. 5,610,524, or in U.S. Pat. No. 5,105,154.
A device allowing such displacement experiments to be carried out at a fixed flow rate is for example described in French Patent 2,708,742 and corresponding U.S. Pat. No. 5,679,885, and a device allowing such experiments at a centrifugation-imposed pressure is described in French Patent 2,758,881 and corresponding British Patent 2,322,942. Furthermore, French Patent 2,778,449 describes a device allowing carrying out such resistivity measurements under reservoir conditions while measuring and controlling the capillary pressure levels in the rock during drainage phases. This allows plotting the evolution of the resistivity index as a function of the brine saturation of the sample. A ceramic arranged at the sample outlet allows reaching very low saturation levels, which is extremely useful to cover the saturation range of a large number of reservoirs. The resistivity data are then directly used by logging engineers to estimate the initial oil saturations and therefore the amounts of oil in place. Besides, this device is also used to obtain the same type of results during imbibition phases (spontaneous and forced). The brine is then injected into the sample at an imposed flow rate through the ceramic. Several phases with increasing injection rates are generally carried out so as to decrease the oil saturation in the sample down to its residual value. The amount of hydrocarbons is then determined from these electric resistivity measurements. The principle is based on the fact that the resistivity of a brine or oil saturated sample depends on the water saturation because oil is not a conducting fluid.
Evaluation of the flow parameters (krs) and of the electric parameters thus requires different equipment, and therefore different samples for each device. Now, evaluation of the content and of the productivity of an underground reservoir requires coherence between the measurements performed on these various samples.
The method and the device according to the invention allow evaluation of the multiphase flow parameters, the capillary effects being entirely and precisely taken into account, and of the electrical parameters of a porous medium, with the same type of equipment, the same experiment and from the same rock sample taken from this medium.